The ability of human peripheral blood lymphocytes to form uropods is greatly enhanced in the absence of serum. Approximately 45% of these lymphocytes bear uropods in the absence of serum while only about 10% have uropods in the presence of serum. The inhibitory factor present in serum and plasma appears to be a lipoprotein. Its apparent molecular weight in serum is 3.0 x 10 to the 6th power daltons and it appears to be part of the beta-lipoprotein complex; however, the lipoprotein may be separated from beta-lipoprotein by ultracentrifugation and has a density similar to the high density lipoproteins. In the presence of heparin the substance is bound tightly to beta-lipoprotein and floats at a density between 1.040 and 1.063. Delipidated, water soluble lipoproteins of this density prepared by heparin and MgCl2 precipitation with a molecular weight of 8,000 - 10,000 daltons, appears to be related to the inhibitory lipoprotein. These apoproteins have been tentatively identified as apoprotein C (C-I, C-II, C-III). The role of lecithin-cholesterol acyl transferase and lipoprotein lipase in the mechanism of action of the uropod inhibitory factor is presently being investigated.